Jialing JH250 performance upgrade

I'm one of 6 or seven owners of JH250E5 cruisers in Harare, Zimbabwe. I've reduced the rear sprocket to 29 teeth to give a theoretical 140kmh @8500rpm but need a bit more torque. We're at 1500m above sea level, so I need smaller main jets than the 98s fitted, but can't find anything here. Has anyone played with one of these to get a bit more power? They are a clone of Honda's CB250, which is rated at 15Kw - but theJialing version only produces 12Kw. If no-one's attacked their motor yet, I'll carry on and keep you informed of progress.

Yo Gonzo... :icon10:

I've had Chinese bikes up to 5000m/asl and can say that rejetting will only have a limited effect. If you'r ehaving trouble with keeping the bike running (like it won't even idle without the idle speed adjsuter waaay up) then the jetting may help but otherwise less atmospheric pressure is less atmospherice pressure and the power will be limited regardless when compared to sea level.

Usually the reason for these engines to run weaker than their JDM counterparts is carburation to pass various emissions inspections. One could always try a carb rack from a Honda on the Jialing and see if it makes a difference.

There's actually not a whole lot of info on here about the JH250 cruisers so you thoughts, opinions and experiences are very welcome. Please keep us posted on how it goes!

CC

I can chime in a little, as I have a Honda Rebel, the bike that you bike is based on.

Firstly, some have one carb, some have twin carbs.

If you have a mikuni style carb on the bike, you may be able to simply richen the mixture by leaving the choke on a fraction. I know it sounds strange, but the choke in MIkuni carbs does not cut off air (and therefore is not a choke, as such) but simply is a richening circuit. Try running the bike up to highway speed, then slowly apply the choke. Figure out where it (the bike) appears to have the most power and use that setting. Make sure you pull the plugs to ensure they are not fouling.

You lack of power may not be related to altitude. Check the plugs for a lean mixture to answer that question.

Thanks to iamcanjim for the Rebel-derived info - I see you guys have a huge site. My problem is not too lean a mixture, but too rich. Even with the head-work the beast still runs rich at full throttle. I'll find some smaller main jets somewhere, but there are limited suppliers here in Zimbabwe. The only other hassle I have is the quality of drive-chains - I've destroyed 2 in 6000 kms (and I do lube them and adjust them). One broke on 4000 ks and the next shed its rollers. Obvious answer is proper o-ring chains but I'm being quoted +/- US$100 a time! Just done an 1100km trip in company with half-a-dozen others, none on less than 750cc bikes, and it held its own, unless there was a headwind or a steep slope. I mounted a GPS for the trip, and recorded a max true speed of 120kmh and an average moving speed of 80kmh, which aint bad! I'm still enjoying, which is the main thing.

O-ring chainz

Good quality o-ring chains in the US can be well over 100USD but certainly you can find some more inexpensive ones here for around 60 or so. Are you using a 528 chain on your bike or a 520?

Runnin' Rich
Have you tried taking out the air filter and seeing how it runs unrestricted? I certainly wouldn't suggest running like this for long but it could be worth trying for effect.

To make the bike lean out a bit a wide open throttle, you could also try lowering the needle one slot, then adding a tiny washer/spacer to effectively lower it 1/2 slot from original (or any increment therein). Since I think the pilot/idle circut is different on the Mikunis it shouldn't affect idle.

G'Luck!
CC

Sorry, obviously my reading comphrehension is poor. High altitude: of course it's too rich.

Leaning it out is easier. Cutting away the airbox (leaving just the air filter) will help a bit. I am never an advocate of loud bikes, but somehow derestricting the exhaust will help a lot. Hammering a screwdriver in the end through some of the baffles might do it. Don't hammer all of them.

So, twin or single carbs?

Btw, i have seen a few Jialing Rebel clones here. They are identical to my bike, so if yours is anything like a China home market Jialing you should have 100% parts commonality with a Honda Rebel.

Keep in mind that US/Canada Honda Rebels and Asia/Australia Honda rebels are different. The Jialing is a copy of an Asian Rebel, so both pipes should be on the right. The front brake should also be on the right. You should have twin carbs (US Rebels have single carbs). You probably have cast wheels rather than spokes.

Everything on the US Rebel boards should work for your bike except carb advice (which is ironically what you are looking for).

Thanks for the info. Zim Jialings are twin-carb and cast wheeled. They arrive with 4 exhausts - 2 being stubs connected through a balance pipe to the headers. I had mine modified to run both on the right, and I have only one baffle plate in each. I've tried running without the air filter but saw no real difference.

This last weekend a South African crew brought their mobile dynamometer up to our little club run - there are no dyno's in Zimbabwe - so I tested it for a laugh. It makes 12,8 kw at 6800, after all the head work and rebore. Then my mate put his absolutely standard, but a year older, bike on and made 12,6 kw! Factory rating is 12 kw. The work I did has hugely improved my performance, so I reckon mine had to be well under factory spec to start with.

I'll keep looking for smaller main jets, and le you all know how it goes.